A belt-alternator-starter (BAS) type hybrid electric vehicle utilizes a motor/generator drivingly connected to an engine crankshaft, typically via a serpentine belt, similar to a traditional alternator. The motor/generator is used to restart the engine when exiting an autostop.
Alternators are typically water cooled or air cooled. Water cooling, typically using a water-ethylene glycol mixture, requires a water jacket surrounding the stator to keep the liquid out of direct contact with the electrical components of the stator. The water jacket adds to the packaging size of the alternator. Furthermore, the water jacket prevents cooling of the rotor. The relatively high temperature of the rotor decreases motor performance, efficiency and durability. Air cooling leaves stator wires exposed to the roadway, risking damage. Oil cooling has been proposed for automotive systems having motor/generators located inside of a transmission casing. Some jets have oil cooled engines from which cooling oil is routed to bearings located on turbines outside of the engine casing.